1. Field of the Invention
This invention relates to a method for controlling an AC motor in which variable-speed control of the AC motor is carried out while power is supplied to the AC motor from a variable-voltage variable-frequency inverter, and particularly to a method for deriving a rotation speed estimate value at the time of idling of the motor.
2. Description of the Related Art
Recently, the methods disclosed in Japanese Patent No. 3,636,340 and JP-A-11-346500 (equivalent to U.S. Pat. No. 6,060,860, which is hereby incorporated by reference) have become known as methods for controlling an AC motor when restarting in a state where a variable-voltage variable-frequency inverter that drives an AC motor (hereinafter simply referred to as inverter) is stopped and the AC motor is idling, for example, when restarting after recovery from a momentary service interruption of a commercial power source or the like that is an input power source of the inverter.
In the control method in which, when the AC motor is idling, at least one of the semiconductor switching devices forming a main circuit of the inverter is turned on and off to generate a short circuit between windings of the motor, and an estimate value of the rotation speed of the motor is calculated from a winding current value flowing at the time, for example, in the control method disclosed in Japanese Patent No. 3,636,340, an induced voltage of the AC motor at the time is used. Therefore, this method can be applied to a synchronous motor having a permanent magnetic field or an induction motor in which a residual voltage exists, but it cannot be applied to an induction motor in which a residual voltage is attenuated.
In the control method in which, when the AC motor is idling, an estimate value of the rotation speed of the motor is calculated from a voltage or current supplied from the inverter to the motor, for example, in the control method disclosed in JP-A-11-346500 (U.S. Pat. No. 6,060,860), if a residual voltage exists in the induction motor at the time, it affects the calculation of the rotation speed estimate value. Therefore, the calculation must be carried out after the residual voltage is sufficiently attenuated.